## Photoelectric measurement

2023, 52(4): 20220862.
doi: 10.3788/IRLA20220862

**As an important internal parameter of the camera, the measurement accuracy of distortion directly affects the image processing accuracy and the geometric positioning accuracy of the camera on orbit. The traditional high-precision laboratory calibration method relying on three-axis turntable has strict requirements for test equipment and test environment. With the increase of camera focus, aperture and volume, this method has increasingly high requirements for equipment and site. The idea of achieving high-precision geometric distortion calibration simply by improving the volume and accuracy of equipment is not applicable. On the basis of the traditional precision angle measurement method, this paper proposes a geometric distortion calibration technology of large aperture and long focal length optical system based on the interference principle. Compared with the traditional precision angle measurement method, this method does not require a high-precision experimental turntable, has good robustness and high accuracy. This paper introduces the basic principle, test method and error link of the distortion test method. The test results of this method are compared with the traditional distortion test method, which shows that the test accuracy of this method meets the requirements of remote sensing camera development and has a wider application range. It can be used for reference for the distortion test of aerospace long focal length and large aperture remote sensing camera.**

**Objective**As an important internal parameter of the camera, the measurement accuracy of distortion directly affects the image processing accuracy and the geometric positioning accuracy of the camera on orbit. At present, the distortion test methods for optical cameras are generally divided into three categories, the high-resolution spaceflight remote sensing camera is more suitable for the test method based on the precision angle measurement theory. However, the precision angle measurement method has strict requirements for the test equipment and the test environment. With the increase of the focal length, aperture and volume of the spatial high-resolution optical system, higher requirements are also put forward for the size and test accuracy of the turntable. It is difficult to realize in engineering and cannot meet the development requirements of various types of space high-resolution remote sensing cameras. Meanwhile, for the space high-resolution optical systems with ultra-large aperture and ultra-long focal length, in order to reduce the influence of gravity in the process of alignment, the vertical method is usually used for alignment. The visual axis of the lens is always perpendicular to the earth in this case, and it is impossible to use the traditional precision angle measurement method to calibrate the distortion of the optical lens in the laboratory. In order to solve this problem, a geometric distortion calibration technology of large aperture and long focal length optical system based on the interference principle is proposed.

**Methods**The whole test system includes laser tracker, special target ball, 4D interferometer, a measured lens, high-precision angle measuring system and plane reflector (Fig.2, Fig.5). In the measurement space coordinate system, the target ball of the laser tracker is placed at the image point (Fig.3). When the center of the target ball coincides with the focus position of the interferometer, the interference self-collimation fringe can be formed (Fig.4). When the target ball is precisely positioned at the image point, the laser tracker can be used to test the image point coordinates to obtain the image height data. The field angle corresponding to the image height of the lens can be obtained by using the high-precision angle measurement system (Fig.6), and the lens distortion value can be calculated by the image height and its corresponding field angle.

**Results and Discussions**Comparative experiments were carried out on optical lens with a focal length of 2 000 mm and a field angle of 2.8° using the traditional angle measurement method and the distortion measurement method based on the interference principle. The calibration results show that the results of the two test methods are highly consistent, and the maximum relative distortion of the two methods are 1.48% and 1.49% (Tab.1). This shows that the method based on interference principle can meet the development requirements of remote sensing camera. A long focal length and large aperture optical lens is tested with the new method. During the test, a total of 21 test points were taken from the full field of view, and three effective tests were conducted (Tab.1). The root-mean-square distortion of the three tests is less than 3 microns, and the maximum relative distortion value is 1.43%. The maximum relative distortion design value of the lens linear array direction is 1.5% (Fig.7), and the test results are in good agreement with the theoretical design value.

**Conclusions**Based on the traditional angle measurement method, a distortion measurement method for aerospace large aperture and long focal length optical system is proposed. This method can meet the distortion test requirements of various types of optical systems. It is used to calibrate the distortion of the traditional optical lens and the large-aperture high-resolution optical lens with vertical adjustment in the laboratory. The results are consistent with the design values, which provides a reference for the development and test of space high-resolution optical remote sensor.

2023, 52(4): 20220676.
doi: 10.3788/IRLA20220676

**Objective**Nano-displacement measurement technology is an important branch in the field of precision measurement, its development and improvement are important guarantee for realizing high-precision nano-manufacturing. With the rise of laser self-mixing interference technology, the precision displacement measurement method with simple structure, low manufacturing cost and measurement accuracy up to nanometer level has been vigorously developed. Laser self-mixing interference technology has been widely used in displacement measurement, absolute distance measurement, speed measurement, and vibration measurement, etc. With the advantages of single optical path structure and the comparable measurement accuracy as double beam interference, the self-mixing interference technology has better application prospect in the industrial area. Traditional laser self-mixing interference schemes mostly take mirrors or scattering surfaces as target mirrors, which take laser wavelengths as measurement benchmarks and are easily disturbed by environmental changes. In order to increase the robustness of the measurement benchmark, this paper studies a laser self-mixing nanometer displacement measurement method based on a planar reflective holographic grating. Different from traditional laser self-mixing interference, the displacement measurement value based on grating feedback is determined by the period of the grating.

**Methods**For the laser self-mixing displacement measurement method based on the plane reflective grating feedback, the vibration displacement value of the holographic grating is reconstructed in this paper. The displacement measurement value of this method is based on the grating period. The system setup is shown (Fig.1). The light emitted by the laser is incident on the grating surface at the Littrow angle, so the retro-reflect one-order diffraction light carry the Doppler phase shift caused by the displacement along the grating period direction. The self-mixing interference output laser is splitted by the structure composed of a half-wave plate and a polarized beam splitter, and the self-mixing signal is collected through a photodetector. In terms of signal processing, the grating self-mixing interference signal is firstly denoised by a low-pass filter and then normalized. Combining the threshold setting method to decide the inversion point of the displacement direction and the phase unwrapping algorithm of arccosine, the displacement of the grating is reconstructed. The grating used in this experiment is a plane diffraction grating with the period of 2400 lines/mm, which equals 416.67 nm. The constructed displacement is compared with the measurement result of a commercial laser interferometer.

**Results and Discussions**In the grating self-mixing interference experiment, the signal under the condition of weak feedback intensity was measured, and the normalized interference signal was shown (Fig.5). After signal processing based on the arccosine method, the corresponding nano-displacement reconstruction results were obtained (Fig.7). The result represents the linear displacement of reciprocating motion as shown in the experiment setting. By calculating the variance of the linear displacement, the entire system has a displacement noise of 5.82 nm, which is expected to be optimized by performing a finer filtering on the signal. From the displacement reconstruction results, the entire measurement result has a linear deviation coefficient of 1.1086 times the actual displacement. A commercial laser interferometer and a grating self-mixing interferometer were also used to compare the displacement measurement data. After the linear correction, the measurement results show that the displacement error does not exceed 0.241% (Tab.2).

**Conclusions**Laser self-mixing nano-displacement measurement method based on the feedback of a planar diffraction grating is studied in this article, and a calculation method using the arccosine method for wrapping phase is proposed. Experimental research was carried out under weak feedback conditions, and the experimental results were reconstructed based on the arccosine method. Compared with the measurement results of commercial laser interferometers, it was found that the laser self-mixing interferometry method based on planar diffraction grating feedback could be used as an effective scheme for nano-displacement measurement. In the future, the measurement accuracy and precision of the grating self-mixing interferometer can be further improved by optimizing the geometric alignment, adopting a more accurate grating, and performing more effective filtering on the signal.

2023, 52(4): 20220646.
doi: 10.3788/IRLA20220646

**Objective**There are a large number of high-aspect-ratio structures in silicon-based MEMS devices, and non-destructive testing of linewidth and depth of these structures is a hot issue at present. Generally, the depth-to-width ratio of MEMS high-aspect-ratio structures is generally between 10∶1 and 100∶1, and the trench width is a few microns to tens of microns. At present, in the silicon-based MEMS process line, anatomical testing is the main means of high-aspect-ratio structure testing, but there are the following defects: it is necessary to use scanning electron microscopy (SEM) for auxiliary measurement, which is inefficient and cumbersome; It is a destructive measurement that causes irreversible damage to MEMS products; It can only be sampled and cannot fully reflect the characteristics of the process. Based on this, a non-destructive measurement system with high-aspect-ratio structure near-infrared broad-spectrum microscopy measurement system was developed, and its measurement accuracy will directly affect the overall performance of the device under test, so it is necessary and urgent to calibrate the measurement system.

**Methods**In order to achieve the accurate calibration of the non-destructive measurement system of high-aspect-ratio structure, a series of standard samples of high-aspect-ratio trenches are designed and developed by semiconductor process, with a width range of 2-30 μm, a depth range of 10-100 μm, and a maximum high-aspect-ratio of 30∶1 (Tab.1). The samples were characterized and fixed, and finally the developed standard samples were applied to the calibration of the near-infrared broad-spectrum microscopy measurement system (Fig.13).

**Results and Discussions**In order to meet the calibration function of the standard samples, a variety of characteristic structures are designed (Fig.1), including auxiliary fixed value structure, measurement positioning structure and positioning angle structure, etc., and the characterization and assessment method of the sample value are designed (Fig.5-6). Measurement values include line width size, trench depth size, and uniformity. Finally, the developed standard template is applied to the near-infrared broad-spectrum microscopy measurement system to further verify the accuracy of the developed system, that is, the applicability of the template (Tab.2).

**Conclusions**In order to solve the calibration problem of the near-infrared broad-spectrum interferometric microscopy system, a series of standard samples of high-aspect-ratio grooves were developed, with a width range of 2-30 μm and a depth range of 10-300 μm, and its high-aspect-ratio reached a maximum of 30∶1. In order to meet the calibration function of the template, a variety of characteristic structures are designed, including auxiliary fixed value structure, measurement positioning structure and positioning angle structure, etc., and the characterization and assessment method of the sample measurement value is designed. Since there is no suitable measuring instrument to directly characterize the value of the standard template of the composite high-aspect-ratio trench, an auxiliary fixed value structure is designed for the standard template, and the cross-section of the high-aspect-ratio trench structure is displayed by sectioning, and then it is measured by scanning electron microscope or atomic force microscope, and the uniformity of the template is characterized to ensure the consistency of the measurement results of the template. Finally, the developed standard template was measured by the near-infrared broad-spectrum interferometric micrometry system, and the measurement results showed that the magnitude was basically consistent with the characterization results.

2023, 52(4): 20220686.
doi: 10.3788/IRLA20220686

**Objective**Cutting occupies a dominant position in mechanical manufacturing, which is the key factor in the development of aerospace, automotive, and electronic industries and other fields. As one of the most important terminals in manufacturing, cutting tool plays an outsized role in removal machining. It is proved that the geometrical parameters have a significant influence on the quality of the workpiece, the efficiency of cutting and the tool life. Given the importance of tool geometry parameters, it is essential to measure these parameters precisely before and during manufacturing. With the increasing requirements of different materials to process, the structures of the cutting tools used to machine become more complex than traditional cutting tools which brings a challenge for the precision measurement. Compared with the common method based on feature images and structured light sensors, the measurement method based on depth from focus presents the advantages of high precision and plenty of data. Hence, a geometry measurement method of cutting tools based on the depth from focus is proposed and the key technique is discussed.

**Methods**Firstly, to solve the problem that the existing focusing function is not suitable for the sequence images of the measured cutting tools and improve the accuracy of the 3D reduction, an improved focus evaluation function based on the double-threshold Tenengrad function is proposed. Due to the surface irregularities of complex tools, the cutting edge is interrupted, resulting in sharp and diverse edge properties. In order to boost the gray gradient and calculate the edge information that the original convolution operator ignored, the function is improved by adjusting the convolution operator and expanding the direction angle of the edge gradient. Moreover, noise information is taken down using an image preprocessing algorithm and a double threshold constraint to generate a high-quality 3D point cloud. Specifically, Gaussian filtering and the Laplacian image enhancement algorithm are applied to remove the image's natural noise while maintaining all of the image's feature information. The threshold determined by the average value and dispersion degree of the gray gradient is then used to reduce noise from the 3D point cloud. After determining the improved function, the properties of no obvious texture on the tool surface serve to establish the size of the computation window for the function. Secondly, the 3D reduction method of the tool flank is optimized using the image processing algorithm, and a technique of measuring the geometric parameters of the tool flank based on vector arithmetic is proposed. To obtain high-contrast sequence images of the tool surface appropriate for calculation, an image enhancement algorithm based on the adaptive sigmoid function is used. Next, the processed sequence images of the 3D point cloud of the tool flank are produced by using the improved focusing function. Additionally, the cutting tool end face parameters defined by the space measurement plane are described and calculated using the vector angle formula and geometric relationship. Besides, it is necessary to carry out plane fitting on the 3D point cloud of the flank based on the RANSAC (Random Sample Consensus) algorithm. Finally, premised on the previous measurement method, a 3D measurement system is built. Utilizing a ladder formed from standard gauge blocks, the system's depth reduction accuracy is confirmed. Conjointly, a comparative experiment is conducted to measure the geometric parameters of the complex tool end face using a variety of measuring systems and techniques.

**Results and Discussions**Throughout every experiment that was done, to test the effectiveness of the improved focus evaluation function, different tool surface sequence images are collected, and focus evaluation function curves related to pixels and the whole image are computed by various focus evaluation functions. The results show that the improved focusing function curve is steeper than other function curves intuitively (Fig.4). The sharpness ratio, steepness, sharpness change rate, and local fluctuation are employed to provide a more objective assessment. It also indicates that other functions pale in comparison to the improved focus evaluation function. The average value is 2 082.9%, 5.4%, 6.7%, and 25.7% better than the Tenengrad function, respectively, according to the index order (Tab.1). Furthermore, it is proved that the depth reduction error of the built system is 0.32% (Fig.8). Eventually, the system and measurement method's data for parameter evaluation reveal that the diameter measurement error is less than 3 μm and the apex angle measurement error is less than 0.3° (Tab.3). It is superior to the Tenengrad function's measurements of the angle (less than 1.9°) and diameter (less than 13 μm). In conclusion, the 3D measurement method based on depth from focus can precisely quantify the geometrical characteristics of cutting tools.

**Conclusions**In this study, an improved double-threshold Tenengrad focusing evaluation function is proposed, which is more suitable for the depth calculation of tool surface sequence images. And the sigmoid function-based image enhancement algorithm is performed to enhance the contrast of sequence images, effectively improving the efficiency and accuracy of the calculation of depth. Further, a prototype of three-dimensional measurement of tool geometric parameters is constructed, and high-quality 3D morphology reconstruction of the tool surface morphology is obtained. Besides, a method for measuring tool geometric parameters based on vector calculation is proposed, and the apex angle and diameter of the tool's inner and outer edges of the main cutting edge are measured. The measured results are within the tolerances of a length measurement error of no more than 10 m and an angle measurement error of no more than 0.5°.

2023, 52(4): 20220732.
doi: 10.3788/IRLA20220732

**Objective**The static angle measurement error for an optical electronic theodolite is generally measured by the star calibration method in the shooting range. Affected by the change of atmospheric refractive index, the empirical formula of atmospheric refraction error is usually used to correct the angle measurement data in the pitching direction of stars. However, the difference between the atmospheric composition in various areas and over time leads to a significant error in the empirical formula. This error results in a significant error in the pitch angle measurement results obtained by the star calibration procedure and affects the separation of other error factors. Therefore, the correction of atmospheric refraction error, which is obtained by the empirical formula, is very important to calculate the total error in measuring the pitch angle of an optical electronic theodolite. To precisely correct atmospheric refraction error, it is usually necessary to use sounding balloons or meteorological aircraft to collect atmospheric parameters at different altitudes. But this traditional method is complicated to organize and difficult to implement. For this purpose, a new method for correcting the empirical formula of atmospheric refraction error is proposed in this paper.

**Methods**A method for correcting the empirical formula of atmospheric refraction error is built in this paper. The method is based on the correlation analysis of star measuring data from multiple theodolites. According to the residual error model for measuring the azimuth and pitch angle of the theodolite, the atmospheric refraction error correction model is derived. Based on this error model, using the residual data of the pitch angle from multiple photoelectric theodolites distributed at different points in the same area, the coefficient for correcting atmospheric refraction error is obtained by fitting the pitch angle measurement residuals and the tangent of the pitch angle with the least square method, and the residual pitch angle data are corrected with the atmospheric refraction error correction model.

**Results and Discussions**Analysis results based on the data of six phototheodolites distributed in the same area show that there were obvious components of the pitch angle measurement residuals that varied linearly with the tangent of the pitch angle (Fig.1). According to the atmospheric refraction error correction model, the pitch angle measurement residuals of the six phototheodolites were linearly fitted to obtain six correction coefficients (Tab.1), and the average value was taken as the comprehensive correction coefficient. After using the comprehensive correction coefficient to correct the atmospheric refraction error, the correlation between the pitch angle residuals and the tangent of pitch angle was significantly decreased (Fig.2), and the total static angle measurement error of the pitch angle of the six devices was significantly reduced (Tab.1). Moreover, before the atmospheric refraction error correction, the peak values of the normalized correlation curves (Fig.6) of the azimuth and pitch angle measurement residuals of each phototheodolites were generally under 0.7 (Tab.2). After the correction of atmospheric refraction error, the peak values of the normalized correlation curves of the azimuth and pitch angle measurement residuals were greater than 0.7, indicating that the correlation of azimuth and pitch angle measurement residuals is enhanced, and this correlation is mainly caused by the correction error of vertical axis tilt angle.

**Conclusions**A method for the correction of atmospheric refraction error based on the correlation analysis of star measurement residual data is proposed. The atmospheric refraction error correction model is derived based on the residual error model for measuring the azimuth and pitch angle of the theodolite. With the residual data of the pitch angle obtained from multiple photoelectric theodolites distributed at different points in the same area, the coefficient for correcting atmospheric refraction error is obtained by the least square method and the residual pitch angle data are corrected. As a result, the total error in measuring the pitch angle of an optical electronic theodolite is significantly reduced after correction of the atmospheric refraction error, and the features of the azimuth and pitch angle residuals caused by the error in correcting the vertical axis tilt error are revealed. The proposed method makes it possible to correct residual pitch angle data of multiple optical electronic theodolites distributed in the same area without using sounding balloons to obtain atmospheric parameters, and the corrected data can be used to separate other error factors, which is of high engineering application value.

2023, 52(3): 20220618.
doi: 10.3788/IRLA20220618

**Objective**Flying target pose estimation is the key technology to realize trajectory prediction and missile guidance control. Real-time calculation of missile posture is conducive to judge whether the missile hits the target, timely detect missile failure, and carry out early destruction. The development of information and intelligent technology has led to the improvement of data acquisition accuracy of color camera, laser radar and other sensors, which has formed a technical system of data acquisition by sensors and target position and attitude estimation by relevant algorithms. Most of the existing target pose estimation methods can effectively detect and estimate the target pose. However, there are some problems in the precision prediction and guidance control of missile landing point, such as the inability to quickly and accurately extract and estimate the position and attitude of the flying target in the complex background. Therefore, on the premise of ensuring being real-time, a method for estimating the pose of flying targets based on area array lidar and bi-modal information fusion is proposed.

**Methods**Firstly, the coordinate transformation model between the camera and the lidar is established to realize the pixel level matching of the two sensors, fusing the image and point cloud at the same time (Fig.2); Secondly, the ViBe (Visual Background Extractor) algorithm and depth information fusion algorithm are used to extract the moving target in the image, and select the corresponding point cloud according to the image moving target position box (Fig.5); Finally, the PnP (Perspective-n-Point) algorithm is used for rough registration of feature points (Fig.8), to obtain the initial rotation and translation matrix between point clouds. And using I-Kd Tree (Incremental K-dimensional Tree) to accelerate the search of adjacent points, the ICP (Iterative Close Point) algorithm is used for accurate registration, to improve the registration speed.

**Results and Discussions**Simulation test and hardware-object simulation test are used to verify the accuracy and stability of the method. The results show that the accuracy of the two-dimensional image object detection algorithm is 97% (Tab.3), and the error classification ratio is 0.0112% (Tab.3). Compared with the traditional ICP algorithm, the accuracy of the pose estimation algorithm is improved by 53.2% (Tab.2), the single time consumption is reduced to 132 ms from 261 ms (Tab.2). Compared with other algorithms, the pose estimation algorithm also has certain advantages.

**Conclusions**An algorithm for estimating the pose of flying targets based on bi-modal information fusion is proposed, which can effectively estimate the pose of flying targets on the basis of selecting appropriate parameters. The accuracy of the algorithm is verified by simulation tests. 50 frames of data are simulated and the average error is calculated under the initial condition that the initial object distance between the target and the lidar is 30 m. The simulation results show that the

*X*-axis error is 1.06 mm, the

*Y*-axis error is 4.59 mm, the

*Z*-axis error is 2.07 mm, the

*Y*-axis rotation angle error is 0.63°, the

*Z*-axis rotation angle error is 1.01°, and the solution time is 132 ms. The accuracy of the algorithm is verified by semi-physical ground experiments, and the precision (

*P*) in the image target extraction test is 0.97, the recall (

*R*) is 0.844, and the percentage of wrong classification (PWC) is 0.011 2%. The statistical average error in the pose estimation test is respectively 4.9 mm for

*X*-axis error, 2.7 mm for

*Y*-axis error, 4.62 mm for

*Z*-axis error, 0.97° for

*Y*-axis rotation angle error and 0.89° for

*Z*-axis rotation angle error. The defect that the single source data of the proposed method is difficult to describe the moving target comprehensively can be remedied, and an objective solution is provided for the position and attitude estimation of the flying target. The proposed method is applied to the accurate prediction and guidance control of the landing point of flying targets, and has high military application value.

2023, 52(3): 20220574.
doi: 10.3788/IRLA20220574

**Objective**Three-dimensional trajectory measurement is a key technology involved in intelligent monitoring, motion analysis and target tracking, which has been widely used in transportation, military and other fields. In recent years, with the rapid development of computer vision technology, imaging equipment and computers are used to replace human eyes and brains to measure the three-dimensional trajectory of target objects with high accuracy. Monocular vision mostly estimates the depth distance of the target in the three-dimensional coordinate system through the proportion of pixel area changes. When the target object rotates and deforms, the depth estimation results are greatly affected. However, binocular vision based on 3D reconstruction mathematical model and polar constraint has the advantages of reliable calculation results and relatively high measurement accuracy in 3D trajectory measurement of flying objects. In the three-dimensional trajectory measurement based on binocular vision, the high-precision matching of binocular homonymous points is the key to improve the measurement accuracy. Especially in the narrow and long space near distance measurement scene for aeroengine safety monitoring, because the binocular camera shoots the target object from different angles, especially when the included angle of the optical axis of the binocular camera is large, the trajectory measurement accuracy of only centroid positioning matching is not high. In order to solve the above problems, a near distance trajectory measurement system in narrow and long space based on centroid matching optimization is developed. In the three-dimensional trajectory measurement based on binocular vision, the high-precision matching of binocular homonymous points is the key to improve the measurement accuracy. Especially in the narrow and long space near distance measurement scene of aeroengine safety monitoring, because the binocular camera shoots the target object from different angles, especially when the included angle of the optical axis of the binocular camera is large, the trajectory measurement accuracy of only centroid positioning matching is not high. To solve these problems, a trajectory measurement system based on centroid matching and optimization is developed.

**Methods**First of all, on the basis of only using the centroid method to locate and match the object, the epipolar constraint projection is used to locate the centroid of the binocular. Then, a gray cross correlation method based on distance and method weight is proposed for subpixel matching of binocular centroids. Finally, Kalman filtering is used to correct the 3D reconstructed motion trajectory of the object, in order to improve the measurement accuracy of the trajectory, the three-dimensional trajectory points with large deviation from the ideal trajectory position caused by the unstable centroid position in the extraction of the target centroid are removed from the three-dimensional trajectory. In the laboratory environment, simulate the narrow and long movement space before the bird enters the engine, build a binocular measurement system at the side close position, and carry out the narrow and long trajectory measurement experiment verification.

**Results and Discussions**According to the measurement experiment results of different texture target objects (Fig.11, Fig.12, Fig.13 and Tab.1), it can be seen that the depth of the target object's imaging texture has a certain impact on the trajectory measurement accuracy of the measurement system in this paper. Because the gray value distribution of the target object with deeper texture is more abundant, the sub-pixel matching based on gray level cross-correlation has better binocular matching effect, so it has higher measurement accuracy. According to the repeatability experiment results (Fig.14), in the full range of 128 mm, the average trajectory length measurement error of the trajectory measurement system in this paper is 13.14 μm for objects with good texture. The measurement accuracy of track length is about 0.01%, and the straightness error of track is small.

**Conclusions**Compared with only using centroid method for coarse positioning and matching, the trajectory length measurement accuracy and straightness of the measurement system are significantly improved, and the high-precision measurement of flying object trajectory in the narrow and long space near distance measurement scene is realized. Based on the error analysis of the measurement results, in the actual measurement, the imaging clarity of the target object texture should be improved by improving the light source illumination and optimizing the optical path design, so as to improve the measurement accuracy of the target object trajectory. The follow-up work direction is to optimize the texture of the target object through image enhancement, improve the trajectory measurement accuracy of the measurement system for the target object with poor texture, and further study the high-precision extraction method of non-rigid body and rotating target matching points, so that the entire measurement system has better stability for the trajectory measurement of different target objects in different measurement scenes.

2023, 52(3): 20220554.
doi: 10.3788/IRLA20220554

**Objective**Infrared target simulator is an important part of infrared target simulation experiment. When the outgoing pupil of the collimation system coincides with the incident pupil of the detection equipment, it can provide a stable infinitely far simulated target for infrared detection equipment, and the simulation results have the advantages of being accurate, controllable and repeatable experiments, which are used to evaluate the performance and accuracy of infrared detection equipment. It has important applications in radar testing, infrared guidance, infrared tracking, etc. With the development of photoelectric detection equipment sensor integration and miniaturization, multi-band sensors have become the standard configuration of most photoelectric detection equipment. Due to the changes in the debugging environment and the use of the environment, it is necessary to adjust it frequently, but most of the target simulators in the laboratory are only equipped with a single-band light source, large size is not convenient to carry. Therefore, it is necessary to establish multi-band and small-sized portable target simulators to meet the needs of different usage environments. For this purpose, an off-axis reflective infrared target simulator system is designed in this paper.

**Methods**A portable infrared target simulator system is built in this paper. A 110 mm aperture parallel light tube of reflective structure was chosen as the collimation system (Fig.2). The optical-mechanical thermal integration analysis of the system was performed to determine the deformation variation of the primary and secondary mirrors and mechanical structure caused by temperature difference (Fig.8). The self-collimating interferometric detection method was mounted using a Zygo interferometer (Fig.11), and the mounting results were judged by the PV and RMS value results of the face shape measurement of the standard plane mirror (Fig.13).

**Results and Discussions**The portable infrared target simulation system was mounted using self-collimating interferometry, with PV value of 0.356

*λ*（

*λ*=632.8 nm）and RMS value of 0.047

*λ*(Fig.13), which is better than

*λ*/20, and the results are excellent and meet the usage requirements. The results of Zernike coefficient analysis shows that the system aberrations are mainly out-of-focus, tilt and higher order aberrations of more than 5 levels (Tab.5), and the adjustable target disc is designed to compensate and improve the imaging quality. A portable infrared target simulator system is built in the laboratory to test the optical path and verify the imaging function of the system. The infrared camera and head were placed at a distance of 10 m from the system, and the imaging results are shown (Fig.14). The targets of different shapes can be clearly identified, and the imaging function of the system has completely satisfies the demand of simulating targets at infinity.

**Conclusions**A portable infrared target simulatot system with working wavelengths of 3-5 μm and 8-14 μm is designed. The system is characterized by simple structure, adjustable wavelength, rich target and clear and stable imaging. The wavefront quality of the system was analyzed using Zemax software, and the PV value of the central field of view was 0.013 2

*λ*and the RMS value was 0.003 8

*λ*in the 4 μm band, and the PV value of the central field of view was 0.004 4

*λ*and the RMS value was 0.001 3

*λ*in the 12 μm band. An optical-mechanical thermal analysis of the collimation system was performed, and at a temperature difference of 30 ℃, the deformation caused by the mechanical structure of the displacement of the optical element is much larger than the deformation of the primary and secondary mirrors themselves, reaching the order of 10 μm, and the imaging results have obvious out-of-focus errors, which can be compensated for the out-of-focus errors introduced by the temperature change by refocusing the target disc with adjustable three-dimensional position. The imaging function of the system was tested, for different shapes of targets, the system can become a clear and identifiable image, providing a stable simulated target for infrared detection equipment.

2023, 52(2): 20220338.
doi: 10.3788/IRLA20220338

In order to improve the quality of automatic fiber placement and assist on-site personnel to quickly detect defects, this paper proposes a real-time instance segmentation network named Trans-Yolact, which is based on Transformer. The Trans-Yolact is used to detect, classify and segment multi-spectrum images of composite material defects. Based on Yolact, aiming at the characteristics of composite material defects, Trans-Yolact's detection ability of composite material defects is enhanced from the two dimensions of space domain and channel domain. In the spatial domain, the convolution kernels have the limitation of spatial scale. The detection of narrow, long, large-size defects is not effective. Therefore, this paper adopts the BoTNet of the CNN+Transformer architecture as backbone; at the same time, the Transformer is introduced into the FPN structure of the Yolact network to enhance the network's ability to obtain information from non-local spaces. In the channel domain, the infrared and visible simultaneous detection method is adopted, and the shallow structure of the backbone is improved, which is divided into visible channel, infrared channel, and mixed channel. Channel domain attention mechanism is introduced in mixed channel. Enhance the comprehensive judgment ability of the network for infrared and visible images. The results show that the mAP of Trans-Yolact for composite defect detection is 88.0%, which is 5.5% higher than Yolact network, and the AP of narrow defects such as miss and twist are increased by 15.2% and 5.1%. The AP of foreign defects including some large-scale defects is increased by 9.1%. Finally, the Trans-Yolact network is pruned. After pruning, the amount of floating-point operations per second (FLOPs) and parameters are reduced by 26.5% and 44.7% compared with Yolact network. The number of detection frames is increased by 58%, reaching 57.67 fps. And the online test is carried out on the large-scale gantry composite material automatic laying equipment, which can meet the real-time detection and segmentation of composite material defects under the maximum laying speed of 1.2 m/s in the production process.

2023, 52(2): 20220593.
doi: 10.3788/IRLA20220593

In order to improve the measurement accuracy, stability and efficiency of the existing 3D coordinate positioning technology, a deep-learning-based point-diffraction interferometer for 3D coordinate measurement method was proposed. A deep neural network was designed for coordinate reconstruction of the point-diffraction interference field. The phase difference matrix was used as the input to construct the training dataset, and the coordinates of point-diffraction sources were used as the output to train the neural network model. The well-trained neural network was used to process the measured phase distribution initially and the phase information was converted to the coordinates of point-diffraction sources. According to the obtained coordinates of point-diffraction sources, the initial particles of the particle swarm optimization algorithm were further modified, and then the high-precision three-dimensional coordinate was reconstructed. This neural network provides a feasible method to establish the nonlinear relationship between the phase distribution of the interference field and the coordinates of the point-diffraction sources, and significantly improves the accuracy, stability and measurement efficiency of the 3D coordinate positioning. In order to verify the feasibility of the proposed method, numerical simulation and experimental verification were carried out, and different methods were used for repeated comparison and analysis. The results show that the single measurement time of the proposed method is about 0.05 s, and the experimental accuracy can reach the submicron magnitude. The mean and RMS values of the repeatability experiments are 0.05 μm and 0.05 μm, respectively, which proves the feasibility of the proposed method and its good measurement accuracy and stability. It provides an effective and feasible method for 3D coordinate positioning.

2023, 52(2): 20220488.
doi: 10.3788/IRLA20220488

The resolution and light collection ability of telescope were directly proportional to its aperture. With the increasingly strict requirements of human beings for the resolution of telescopes, the size of telescope mirrors would be also increasing. With the increasing size of the mirror, the mirror seeing became more and more important. Mirror seeing mainly referred to the degradation of image quality caused by turbulence on the mirror surface. When the mirror size exceeded the local atmospheric turbulence scale, we had to consider the influence of this factor on imaging or processing. The working environment of the system would affect the mirror seeing to a certain extent, so the mirror seeing was also of great significance to the integrated detection process. Therefore, in order to improve the surface accuracy of mirror processing and the integration effect of the detection system, it was necessary to accurately measure the mirror seeing of the instrument, so as to provide judgment for its processing detection and application integration. In our work, one-dimensional detection (autocollimator method, etc.), two-dimensional detection (slope/curvature method, holographic wavefront sensing method and shearing interference method, etc.) and three-dimensional detection (holographic particle velocimetry and temperature field method, etc.) were described from three aspects: principle, research status and application in mirror seeing. By introducing the detection methods for different scenes and detection requirements, it had a good guiding significance for the detection of mirror seeing.

2023, 52(2): 20220367.
doi: 10.3788/IRLA20220367

The planar-array-based imaging radar can achieve transient 3D detection and is suitable for pose measurement of moving platforms or non-cooperative targets. A multi-view point cloud auto-registration method for pose measurement of spatially non-cooperative targets was proposed for non-uniform grid point clouds with crosstalk characteristics between adjacent pixels. Based on the principle of improved coherent point drift (CPD), the method treats the target point cloud as the data distribution set and the source point cloud as the set of center-of-mass points of Gaussian mixture model (GMM). The likelihood function of the constructed GMM model is solved by using Bayesian posterior probability formula and Expectation-Maximum (EM), and the weight of the point set are adaptively adjusted by the overlap of the point clouds in the optimization process. The distance residuals between source point set after one EM iteration are ranked, the optimal transformed point cloud pair is selected, and the local perturbation quantity is established using the nearest neighbor method to obtain the spatial transformation matrix for each drift iteration. To avoid getting into local solutions, the attributes of the point set involved in the drift operation are alternated by supervising the mean square error update rate of the point cloud. For spatially targets, two simulation conditions are established to obtain multi-view non-cooperative target point cloud datasets. The results show that the method is robust under the strong noise and pixels blurring interference, and the average largest common point set corresponding is improved by approximately 61% compared with the other coarse-fine registration strategy, which can be applied to the non-cooperative target pose measurement under the spatial planar-array-based 3D imaging platform.

2023, 52(2): 20210813.
doi: 10.3788/IRLA20210813

With the development of photoelectric measurement technology, infrared gas detection technology is widely used in many fields. Temperature has an important influence on the detection of gas concentration and isotopic abundance. The traditional temperature control system using proportional integral differential (PID) control algorithm has the disadvantages of overshoot, slow response time and low precision. Firstly, COMSOL software is used to determine the heating structure by finite element analysis. Secondly, the STM32 single chip microcomputer is used to collect real-time temperature data through 16 bit AD chip LTC1864. Finally, the linear auto disturbance rejection algorithm (LADRC) is used to adjust the PWM wave that achieve the high-precision and real-time dynamic adjustment of the system temperature by controlling the semiconductor cooler (TEC). Under the temperature of 19.8 ℃ condition, an temperature control experiments with a target temperature of 32 ℃ is carried out. The results show that the standard deviation of temperature fluctuation is 0.0357 ℃. Compared with the temperature control system using PID algorithm, it has the advantages of no overshoot, fast response time and high precision.

2023, 52(2): 20220451.
doi: 10.3788/IRLA20220451

To improve the spectral matching accuracy of the star simulator light source system, firstly, a star simulator light source system based on digital micromirror is designed and built. Secondly, the spectral fitting of the genetic algorithm is performed according to the regional spectrum calibrated by wavelength, The results show that the scheme exits a certain matching error between the fitted spectrum and the target spectrum. Finally, in order to improve the accuracy of spectral matching, an error feedback and accuracy improvement method is proposed to divide the region into two-dimensional wavelength and energy. The experiment simulates light sources with color temperatures of 2550 K, 4766 K, 6576 K, and 8910 K. The results show that, compared with the feedback method of one-dimensional division in the wavelength direction, the maximum error of spectral matching decreases by 55.7%, 50.6%, 45.2%, and 42.2%, respectively, which significantly improves the spectral matching accuracy of the star simulator light source system. The study aims to compensate for the angle measurement error caused by the spectral match error, which improves the star sensor's calibration accuracy.

2023, 52(2): 20220408.
doi: 10.3788/IRLA20220408

With the continuous progress of science and technology, military targets are developing towards miniaturization, ultra-high speed and low detectivity, so the detection and identification ability of targets is also put forward higher requirements. Based on the deep analysis of point target imaging process and on the basis of energy distribution, based on abnormal Rayleigh distribution of point target energy calculation method, the infrared radiation characteristics and has a better precision than the experimental verification, it is concluded that the measurement deviation of radiation intensity can be controlled under 8%, and measuring the degree of discrete numerical smaller, can effectively distinguish between the point target and background. It is proved that the calculation method has good engineering applicability, high measurement accuracy and wide application prospect.

2023, 52(1): 20220278.
doi: 10.3788/IRLA20220278

Vector measurement is an important technology for beam testing of antennas and quasi-optical systems in terahertz band. This paper introduces a terahertz vector measurement system based on a high-sensitivity AlGaN/GaN high-electron-mobility transistor (HEMT) terahertz detector integrated with a quasi-optical lens and waveguide together, which reached the noise equivalent power of −113 dBm/Hz in heterodyne mode at 340 GHz. A hardware circuit is established based on the double frequency-down-conversion technique to suppress phase noise in the system. The experimental results indicate that the minimum measurable power is 119 nW and the phase stability is better than 4° of the system. Measurement of the distribution of both terahertz amplitude and phase has been achieved based on this coherent AlGaN/GaN HEMT detector. An arrayed terahertz vector measurement system could be developed based on this work.

2023, 52(1): 20220339.
doi: 10.3788/IRLA20220339

The Lambertian diffuse reflectance characteristics of the spaceborne solar calibration diffuser and its radiation attenuation characteristics directly determine the long-term accuracy and stability of the on-orbit radiation calibration of space remote sensing instruments. In order to effectively improve the on-orbit radiation calibration accuracy of spaceborne ultraviolet hyperspectral detection instruments, based on the introduction of commonly used solar calibration diffuse reflector materials in the field of space remote sensing, a new type of ultraviolet wavelength diffuser material is proposed: high purity opaque fused silica material HOD, and the diffuse reflection Lambertian characteristics and radiation attenuation characteristics of the new high purity opaque Fused silica HOD diffuser and the traditional aluminum diffuser are compared by testing. The results show that after 32 equivalent solar hours (32ESH) of vacuum ultraviolet irradiation, the attenuation of the high purity opaque Fused silica HOD diffuser at the wavelength of 290 nm is 7.5%, which is better than 10% of the traditional aluminum diffuser. And the Lambertian maximum cosine deviation of the traditional aluminum diffuser around 290 nm is about 40%, while the high purity opaque Fused silica HOD diffuser is about 10%. Therefore, the diffuse reflection characteristics of the new high purity opaque Fused silica HOD diffuse reflector in the ultraviolet band are better than those of the traditional aluminum diffuser. The high purity opaque Fused silica hod diffuser has better diffuse reflection Lambertian characteristics and stronger vacuum ultraviolet radiation attenuation characteristics, so it can improve the long-term accuracy of the on-orbit radiometric calibration of space ultraviolet remote sensing instruments.

2023, 52(1): 20220414.
doi: 10.3788/IRLA20220414

Redundant Rotating Inertial Navigation System (RRINS) can further improve the reliability of the system on the basis of traditional rotating inertial navigation system. Aiming at the high-precision initial alignment requirements of this type of system, A two-position initial alignment method was studied by taking the regular tetrahedral redundant rotating inertial navigation system as an example. Firstly, every three gyroscopes and three accelerometers constituted a combination. The zero bias correlation and redundancy configuration of the inertial device under each combination were established. And the RRINS two-position stop scheme was designed to estimate the zero bias of the corresponding inertial device. But in some special cases, the observation position needs to be increased. Then, the results obtained by each inertial device under different combinations were averaged, and the average value was used to compensate the measurement information of the corresponding inertial device. Finally, based on the compensated inertial device output performs the initial alignment of the RRINS. Mathematical simulation and experimental verification results show that the method can effectively estimate the zero bias of the inertial device under different two-position schemes. In the simulation, the bias estimation error of the gyroscope is within 4%, and the bias estimation error of the accelerometer is basically within 2%. Compared with the case without bias compensation, the initial alignment accuracy is improved by more than 10 times. In the experiment, the initial alignment accuracy in both horizontal and azimuth directions was improved, and the heading angle alignment error was reduced by about 100 times. At the same time, the method can also be extended to redundant rotating inertial navigation systems with other configuration schemes, which has certain reference significance for improving the initial alignment accuracy of such inertial navigation systems.

2022, 51(9): 20210952.
doi: 10.3788/IRLA20210952

In order to solve the problem of limited internal space and difficult measurement of some workpiece, a point cloud rotating splicing method based on surface structured light was proposed. The reconstruction method of single field of surface structured light was introduced in this paper. The absolute phase value was obtained by combining four-step phase shift and complementary Gray code, and the camera and projector were calibrated by polynomial fitting method. The point cloud registration was studied based on the rotation plane of the wrist joint at the end of the manipulator. A calibration method based on the auxiliary camera was proposed, and the transformation relationship between the camera imaging coordinate system and the rotation plane coordinate system was given. The experimental results show that the method is suitable for measuring the inner wall of workpiece, and the average error of splicing is less than 0.05 mm, which meets the requirements of practical application.

2022, 51(9): 20210825.
doi: 10.3788/IRLA20210825

The single photon scattering echo characteristics of targets were studied in this paper. An optical scattering characteristic measurement system was built based on infrared single photon detector and picosecond laser. The number of echo photons was used to characterize the optical scattering characteristics under the condition of single photon detection. In this experiment, the single photon scattering characteristics of targets with different shapes (sphere, cube, cylinder and cone) were studied. And the results were fitted by using the bidirectional reflection distribution function model. The experimental results were in good agreement with the theoretical fitting ones. Further, the single photon scattering characteristics of targets with different materials (ceramic tile, wood and wall brick) were studied, which were compared with the traditional wave scattering characteristics. This study provides a reference for the long-range target recognition and detection of single photon lidar.

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